The purpose of this study is to investigate skin tumor progression by using markers which would permit the follow-up of tumors during different phases of skin carcinogenesis. Since the presence of gamma glutamyl transpeptidase (GGT) together with the decrease or absence of high molecular weight keratin polypeptides have been considered good markers of malignant potential in benign skin tumors, the sequential detection of these markers in tumors of different potentials produced by complete carcinogenesis, two-stage carcinogenesis and three-stage carcinogenesis will permit a detailed study of the malignant conversion of papillomas and keratoacanthomas. GGT will be studied using histochemical and biochemical techniques. The topohistochemical distribution of GGT, as well as the isoelectric focusing patterns, and the relationship of GGT with cell proliferation will be studied in the tumors produced by different protocols at different stages of tumor development. The keratin components will be studied in these same tumors using gel electrophoresis and immunohistochemical techniques. Other parameters which are useful in evaluating altered keratinocyte differentiation, such as filaggrin, plasminogen activator and lectin binding will also be studied in order to evaluate more completely the relationship between gradual altered differentiation patterns and skin tumor progression.